Such a construction, also referred to as a two-pass boiler, is known for example from EP 2 182 278 A1. Here, welded-together steam generator tubes through which a flow medium can flow form both the gas-tight enclosure walls and gas-permeable grate walls of the continuous flow steam generator. Correspondingly arranged collectors connected to the steam generator tubes make it possible to form different heating surface segments composed of groups of steam generator tubes, connected in parallel, of the enclosure walls. In principle, it is possible here for the steam generator tubes of the continuous flow steam generator to be arranged vertically and/or in helical or spiral-shaped fashion in part or over the entire length. Furthermore, the continuous flow steam generator may also be in the form of a continuous forced-flow steam generator.
DE 10 2010 038 885 A1 has disclosed a continuous flow steam generator with vertical tubing, referred to as a single-pass or tower boiler. In this case, the tubing of the enclosure walls is divided into a lower section and an upper section, which are connected to one another by a passage collector. The passage collector duly effects complete pressure equalization between the steam generator tubes without further measures, but effects only an incomplete mixing of the flow medium. Differences in the outlet temperature or outlet enthalpy of the steam generator tubes in the lower section are only partially compensated in the passage collector, and are therefore conducted onward, partially unmixed, to the steam generator tubes in the upper section. Since heating imbalances however also exist in the steam generator tubes of the upper section, local temperature differences of the flow medium in the steam generator tubes can intensify further within the enclosure walls, and can thus under some circumstances reach inadmissibly high values. If the temperature values exceed the scaling temperature of the material, or if inadmissibly high material stresses arise owing to the high temperature values, damage to the enclosure walls can occur, which must be avoided for reliable operation of the power plant.
Therefore, in DE 10 2010 038 885 A1, for a continuous forced-flow steam generator with parallel steam generator tubes in the upper section, it is proposed that the design parameters for said steam generator tubes be selected such that the mean mass flow density in said steam generator tubes at full load of the steam generator does not lie below 1200 kg/m2s. The homogenization of the flow distribution and avoidance of stagnation in the upper vertical tubing which are achieved in this way may however under some circumstances not suffice as a measure for reducing local temperature imbalances to such an extent that conventional materials, such as for example 13CrMo45 (T12), can be used. In such cases, it is then possible for more highly alloyed materials to be used. Accordingly, for the enclosure walls in particular of the upper section, the materials 7CrWVMoNb9-6 (T23) or 7CrMoVTiB10-10 (T24) are discussed or used, wherein, in the case of said materials, for reliable operation of the continuous flow steam generator and of the power plant as a whole, particular attention must be paid to the reliability and durability of the welded connections.